The neocortex varies vastly in size and complexity yet its cytology, laminar architecture, and general plan of cytoarchitectonic organization are closely similar across mammalian species. These similarities of structure and organization emerge in the course of a closely similarly developmental history. Thus, the neocortex in all species arises in the course of a discrete neuronogenetic interval (NI) from a pseudostratified ventricular epithelium (PVE) located at the margin of the developing cerebral wall. Once their terminal cell division in this epithelium has been completed the young neurons migrate across the cerebral wall to the neocortex where they grow, differentiate, and become synaptically incorporated into cerebral neural systems. The neurons forming the deepest cortical layers are the earliest to be formed while progressively later formed neurons arise at progressively later times in development. In experiments in mice, we have determined that it is the relation of total cell cycle number, occurring in the course of the NI, to the cell cycle output function, Q, which is regulatory to the duration of NI and to the rate of neuron production. Cell cycle number appears largely to be regulated by progression in the length of the G1 phase of the cycle. We propose that regulation is mediated by cell external substances, acting upon the proliferating cell during G1 phase.
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